When manufacturing a seamless steel pipe by the Mannesmann mandrel mill process, first, a round or square billet is charged into a heating furnace and heated. Next, the round or square billet undergoes piercing rolling using a piercer to form a thick-walled hollow shell. Then, a mandrel bar is inserted into the hollow shell, and the shell undergoes elongation rolling using a mandrel mill, which typically comprises 5 to 8 roll stands, to decrease the wall thickness to a predetermined value and form a mother tube. The mandrel bar is then withdrawn from the mother tube, and the mother tube undergoes sizing rolling using a reducing mill to give a predetermined outer diameter and thereby manufacture a seamless steel pipe which is a final product.
A mandrel mill for carrying out elongation rolling is typically a 2-roll mandrel mill having two sets of elongation rolls disposed in each roll stand. However, with a 2-roll mandrel mill, the extent of deformation of a hollow shell which is being rolled greatly differs between portions of the hollow shell corresponding to the groove bottoms of the rolls (referred to below simply as the groove bottom portions of the shell) and the portions thereof corresponding to the flange portions of the rolls (referred to below simply as the flange portions of the shell). Therefore, the stress balance in a hollow shell which undergoes elongation rolling in a 2-roll mandrel mill is easily upset, and it is difficult to achieve a high working ratio with a 2-roll mandrel mill.
In recent years, use of a 3-roll mandrel mill in place of a 2-roll mandrel mill, which is difficult to achieve a high working ratio, has been disclosed (see Patent Document 1, for example).
In order to suppress the occurrence of thickness deviations which is a phenomenon in which four locations in the circumferential direction of a hollow shell locally become thickened due to rolling with a 2-roll mandrel mill, it has been proposed to employ a 4-roll stand which performs local reduction of the four locations where the wall thickness is increased as the final stand of a mandrel mill. Patent Documents 2 and 3 disclose rolling techniques and equipment for carrying out such proposal.
However, if roll stands having a different number of rolls are installed in the same mandrel mill, the equipment becomes complicated, and design and improvement of the equipment become difficult.
In a 2-roll mandrel mill, a pair of opposing grooved rolls can contact each other at their roll flanges. Therefore, zero point adjustment of the reduction positions of the rolls can be easily carried out. In contrast, with a 3-roll mandrel mill or a 4-roll mandrel mill, such contact between rolls cannot be achieved. Therefore, zero point adjustment of the reduction position of the rolls is difficult compared to a 2-roll mandrel mill, and it is difficult to guarantee dimensional accuracy after rolling. Patent Document 4 discloses an invention which adjusts the zero point of the reduction positions using actual values measured with a wall thickness gauge.    Patent Document 1: JP 2005-111518 A    Patent Document 2: JP H08-71614 A    Patent Document 3: JP H11-123409 A    Patent Document 4: JP 2005-131706 A